1. Field of the invention
The present invention relates to an AM stereo receiver and, more particularly, to a detector for detecting a very low frequency identification (ID) signal in the received broadcasting signal indicating the presence and also the type of AM stereo system being received. The present invention also relates to such an AM stereo receiver which can maintain the received ID signal even when the signal temporarily fluctuates.
2. Description of the Prior Art
At present, three different AM stereo systems are proposed, and are being actually broadcasted in U.S.A. The first system is a quadrature modulation stereo system known as a Motorola AM stereo system. The second system is an independent side-band modulation stereo system which is known as a Kahn AM stereo system, and the third system is a phase modulation system which is known as a Magnavox AM stereo system.
The frequency of the ID signal of Motorola AM stereo system is 25 Hz, that of the Kahn AM stereo system is 15 Hz, and that of the Magnavox AM stereo system is 5 Hz. Since the ID signal is present whenever the stereophonic signals are being broadcasted, the discriminate between the monophonic broadcasting and the stereophonic broadcasting can be accomplished by the detection of the ID signal. Furthermore, by the detection of the frequency of the ID signal, it is possible to detect the type of the stereo system among Motorola, Kahn and Magnavox AM stereo systems.
The detector circuit for detecting the ID signal, particularly the ID signal of Motorola AM stereo system, is disclosed in U.S. Pat. No. 4,405,837 to Ecklund. According to U.S. Pat. No. 4,405,837, the ID signal of Motorola AM stereo system is detected by the use of a band pass filter for filtering the ID signal of 25 Hz. Therefore, to detect three different ID signals, one method is to prepare three different sets of band pass filters for filtering 25 Hz ID signal, 15 Hz ID signal and 5 Hz ID signal, respectively.
However, the use of the band pass filter, or low pass filter, results in the following disadvantages. Since the filter generally contains one or more capacitors, the filter circuit is not appropriate to be assembled in an integrated circuit. Also, since the frequency of the ID signals of Motorola, Kahn and Magnavox AM stereo systems are relatively close to each other, it is necessary to prepare the three different filters that have a very narrow pass band, i.e., a high quality factor. However, when the quality factor is made high, the center frequency of the pass band may be easily shifted by the deviation of the components employed.